The present invention relates to a method for controlling furnace pressure of a shuttle kiln, and, more particularly, to an improvement of method for controlling pressure in a shuttle kiln to a target value by controlling the number of revolutions of an exhaust gas fan to adjust an exhaust gas flow rate of combustion a exhaust gas or the like.
To maintain a combustion state in a shuttle kiln at the most desirable state, it is one of requirement to control a kiln pressure to within a certain range. The kiln pressure is obtained by providing suitable resistance in exhausting combustion air or fuel supplied into the kiln from burners of the shuttle kiln or other feed air such as cooling air in order to cause a pressure difference between inside and outside of the kiln.
In a shuttle kiln, particularly, since a series of heating operations such as raising of temperature, soaking of the maximum temperature or cooling are performed, supply operations significantly vary for combustion air or cooling air as time passes, so that control of kiln pressure is one of automatic control factors essential to the operation of shuttle kiln.
Herein, is described an example of conventional method for controlling kiln pressure of a shuttle kiln with reference to FIG. 4. In the kiln body 11 of the shuttle kiln, fuel, combustion air or the like fed from outside become exhaust gas after combustion, the exhaust gas being discharged to an exhaust flue 12, fed to a chimney 15 from an exhaust gas damper 3 through a plenum 13, into which outside air is introduced, as required, by an exhaust gas fan 14, and finally discharged into the air.
In this case, to also control the kiln pressure in the kiln body 11, there is formed a PID control loop which uses the kiln pressure as an input element and the number of revolution of the exhaust gas fan 14 as an output element. Referring to FIG. 4, the kiln pressure in the kiln body 11 is sent by the pressure transmitter 21 to a control arithmetic unit 22, which in turn instructs an inverter controller 23 driving the exhaust gas fan 14 to issue a variable speed drive output so that the exhaust gas fan 14 rotates at a number of revolution depending on a deviation from a target value.
Thus, the kiln pressure can be stably maintained by automatically adjusting flow rate of the exhaust gas in such manner that the number of revolution of the exhaust gas fan 14 is decreased under control of the variable speed drive to raise the kiln pressure, and increased to reduce the kiln pressure.
In addition, the flow rate of exhaust gas may be controlled by adjusting opening of the exhaust gas damper 3 with an exhaust gas damper program 31 which is programmed according to elapsed time. However, in operation of an actual kiln, it is unavoidable that offset from the program is caused by disturbance such as a shape of a product, changes in dimensions, pack weight, or variation of outside air temperature. Accommodation to such situation requires corrective operation by an operator, or safety measures to maintain the number of revolution at a value higher than necessary, which causes disadvantages in promotion of automation or energy saving measures.
On the other hand, when the number of revolution of the variable speed drive is controlled for the exhaust gas fan 14, there are disadvantages that, in a range between 1/10 and 1/20 or lower of the rated number of revolution, a motor for driving the fan would have insufficient continuous allowable torque, and could not handle load torque, leading to unstable rotation, and thus difficulty in control, and that cooling air flow becomes insufficient due to insufficient number of revolutions causing insufficient cooling in the motor. Thus, the control range for the number of revolutions of the exhaust gas fan would be values in a range between 1:10 to 1:20 in terms of the rated number of revolution, leading to a problem that the flow rate of exhaust gas can be controlled only for values in a range between 1:10 and 1:20 corresponding to the control range.